1. Field of the Invention
The present invention relates to methods for manufacturing non-oriented magnetic steel sheets having superior magnetic properties. Such sheets are ideally suitable as magnetic components for rotating elements such as electric motor cores and the like.
In particular, this invention relates to methods for manufacturing semiprocessed non-oriented magnetic steel sheets having superior workability in assembling iron cores for motors and the like, and having superior magnetic properties after stress relief annealing following assembly.
As referred to herein, a xe2x80x9csemiprocessedxe2x80x9d magnetic steel sheet is a substantially non-oriented magnetic steel sheet which exhibits superior magnetic properties after stress relief annealing. Usually, stress relief annealing is performed at 700 to 800xc2x0 C. for approximately 2 hours following the step of die cutting the sheet by customers. In general, the non-oriented steel sheet is manufactured by pickling hot-rolled steel sheets with or without annealing thereof, and cold rolling, annealing and skin pass rolling the sheet.
2. Description of the Related Art
Magnetic sheet steel materials for rotating motor or generator cores or the like are magnetized in various directions substantially parallel to the surfaces of the materials. Such materials have substantially no magnetic anisotropy and are accordingly very advantageous for use in rotating electrical components and the like.
A method is disclosed in Japanese Examined Patent Application Publication No. Hei-7-59725 in which hot rolling conditions are controlled, and a method is disclosed in Japanese Unexamined Patent Application Publication No. Hei-3-75313 in which annealing is performed for hot-rolled steel sheets.
In addition, recently, in order to alleviate the effect of magnetic anisotropy of the steel sheets after performance of die cutting operations for cores for motors, manufacturing methods for making iron cores have been somewhat improved. For example, a so-called xe2x80x9crotation pilingxe2x80x9d method can be performed, in which, when a set of a predetermined number of cores is piled, a following set of cores is piled at or along one or more different angles therefrom, by rotating. As a result, differences in performances of motor cores caused by anisotropy of materials are not very significantly manifested, compared to those conventionally observed.
Recently, the processes of assembling motor cores have been significantly automated. As a result, in particular, improvements of material thickness accuracy and of die cutting properties have been strongly desired.
Concerning improvements of material thickness accuracy and of die cutting properties, some methods have been proposed; for example, Japanese Examined Patent Application Publication No. Hei-4-25345 disclosed a method in which grain diameters of a steel sheet are controlled before skin pass rolling, Japanese Unexamined Patent Application Publication No. Hei-9-35925 disclosed a method in which an appropriate amount of titanium (Ti) is added, and Japanese Unexamined Patent Application Publication No. Hei-10-25552 disclosed a method in which material elongation percentage is controlled. However, since the methods mentioned above are proposed from experimental results based on the observed phenomena, the reasons for the proposals are not sufficiently explained, and in addition, any effects achieved are not sufficient for practical commercial use.
An object of the present invention is to provide a manufacturing method which can be advantageously applied to the manufacture of semiprocessed magnetic steel sheets, in which productivity increase and higher accuracy of product thickness can be realized by improving workability of a die cutting step without impairing the magnetic properties of the sheet. Improvements of workability in a die cutting step can be achieved by, for example, reducing flash height and reducing material thickness variation.
According to the present invention, the method for manufacturing a semiprocessed non-oriented magnetic steel sheet having superior workability and magnetic properties after stress relief annealing, comprises hot rolling a steel slab containing about 0.001 to 0.03 wt % carbon (C), about 0.1 to 1.0 wt % silicon (Si), about 0.01 to 1.0 wt % aluminum (Al), about 0.05 to 1.0 wt % manganese (Mn), and about 0.001 to 0.15 wt % phosphorus (P), cold rolling the hot rolled sheet, continuously annealing the cold rolled sheet and skin pass rolling the annealed sheet, wherein the average rapid cooling rate of continuous annealing is about 10xc2x0 C./second or more, and wherein skin pass rolling is performed at a reduction rate of about 0.5 to 5% within about 20 hours after the rapid cooling is completed. In addition, in the method of the present invention, rapid cooling in the continuous annealing step is preferably performed at a rate of about 10xc2x0 C./second or more between about 600 to 400xc2x0 C. Furthermore, the steel slab preferably further comprises at least one of about 0.001 to 0.20 wt % tin (Sn), about 0.001 to 0.10 wt % antimony (Sb), and about 0.001 to 0.010 wt % boron (B), and the difference between the amount of carbon present in the steel slab and the Ceq obtained by the equation (1) shown below is preferably about 0.001 wt % or more, in which the Ceq value is calculated from the wt % amounts of the impurities titanium (Ti), niobium (Nb), vanadium (V), and zirconium (Zr) in the steel slab. The equation is:
Ceq(wt %)=12xc3x97{[Ti(wt %)]/48+[Nb(wt %)]/93+[V(wt %)]/51+[Zr(wt %)]/92}xe2x80x83xe2x80x83(1)